The present disclosure relates to anode half-cell reactions (oxidations of fuel or other reductants) in fuel or other electrochemical cells. More particularly, it relates to expanding the scope of fuels or other reductants that are usable by utilizing a flow-based anode half-cell where the fuel or other reductant oxidation occurs away from the electrode (the anode). This is accomplished by using a carbon-containing redox mediator that is capable of transferring electrons and protons, in combination with a heterogeneous redox catalyst that is not in direct contact with the anode.
Fuel cells are comprised of two half-cells, with an electrolyte separating them that allows for ions to flow. At the anode, a fuel or reductant (with typical examples of fuels including but not limited to: hydrogen, methane, methanol, or biomass) is oxidized, and at the cathode, oxygen or another oxidizing agent is reduced. Electrons flow from the anode to the cathode through an external circuit, and ions flow between the anode and cathode to maintain charge balance between the respective half-cells. The electricity generated from the flow of electrons can be used in a variety of applications, such as for generating primary or backup electrical power in stationary or mobile applications and supplying the electricity needed to power an electric vehicle, such as a forklift or an automobile.
For most conventional PEM (polymer electrolyte membrane) fuel cells, both the fuel (H2) and air or O2 are introduced as gases, and undergo oxidation or reduction, respectively, at gas diffusion electrodes containing platinum. A subset of PEM fuel cells, which use solutions of methanol as the fuel, typically use Pt or Pt alloyed with Ru as electrocatalysts for fuel oxidation. Some other fuels have been considered, such as formic acid; however, H2 and methanol (MeOH) are the most frequently studied fuels. The anode chemistry for conventional PEM fuel cells using H2 as a gas is quite well developed, and this chemistry is not frequently seen as limiting fuel cell usage. H2 has several promising characteristics as a fuel for fuel cells, including its good energy density, its innocuous byproducts, and the potential for being sourced from renewable sources. However, there are also complications associated with using H2 as a fuel. For example, it is a flammable gas that is difficult to store. The infrastructure for H2 delivery is also less developed than the infrastructure for liquid fuels, meaning it can be less accessible or more expensive. For this reason, using a liquid fuel such as MeOH would be advantageous.
Liquid fuels (or solutions of fuels) have the previously discussed advantage of easier distribution than H2, with MeOH being a very attractive fuel. However, a major difficulty in using MeOH is the crossover of MeOH from the anode half-cell to the cathode half-cell, where it poisons the Pt catalysts typically used to reduce O2 at the cathode. Other potential fuel sources, such as biomass, typically contain impurities that are capable of poisoning both the anodic and cathodic electrocatalysts. Due to this poisoning, lower concentrations of fuels are used, which decreases the power output of the fuel cell.
One strategy for decreasing the poisoning due to crossover and to extend the range of fuels able to be oxidized in a fuel cell is to move the fuel oxidation reaction (and, optionally, the O2 reduction) off of the electrode. This strategy uses a redox mediator capable of transferring protons and electrons that can shuttle electrons from the fuel to the anode. The fuel oxidation is assisted by a redox catalyst. The oxidation of the fuel supplies the reducing equivalents capable of reducing the redox mediator, which is then reoxidized at the anode.
There have been previous efforts to move fuel oxidation (and in some of the same systems, O2 reduction) off of the corresponding electrode.
U.S. Pat. No. 3,682,704 discloses a redox anode containing Cu or Ag salts as combined redox mediator/catalysts and sugars as the fuel that is oxidized. The catholyte solution additionally contains Cu, Fe, or Ag as redox mediators for O2 reduction.
U.S. Pat. No. 4,396,687 discloses a redox anode utilizing a silicon-based polyoxometalate redox mediator with a Pt-based redox catalyst for H2 oxidation. Vanadium salts are used as redox mediators in the cathode, with a polyoxometalate-based redox catalyst for O2 reduction.
U.S. Pat. No. 5,660,940 discloses a redox fuel cell using carbohydrates as fuels, a Pt-based redox catalyst, and vanadium salts as the redox mediator in the anode. Vanadium salts are also used as a redox mediator in the cathode, where nitric oxide or a metal phthalocyanine redox catalyst are proposed for O2 reduction.
A fuel cell utilizing viologen-based redox mediators for the oxidation of glucose under basic conditions is reported in U.S. Pat. No. 8,404,396.
A biomass-based fuel cell utilizing polyoxometalates as redox mediators for the oxidation of a variety of biomass is reported in U.S. Pat. Appl. 2016/0,344,055. Polyoxometalate mediators are also used in the cathode compartment.
However, each of these examples suffers from various disadvantages. These factors include high molecular weight of the mediators relative to the number of electrons they can transport, high cost, low stability, inability to tune the redox properties, and insufficient current/power densities.
In U.S. Patent Publication No. 2015/0263371, which is incorporated by reference herein, we disclosed a strategy specific to O2 reduction at the cathode using specific classes of redox mediators in combination with redox catalysts that were not attached to the cathode. Notably, the disclosed strategy would not have been expected to work for the oxidation of a fuel or other reductant in a cathode half-cell.
Accordingly, there remains a need for electrochemical cells having improved anode half-cell performance, for the more efficient electrocatalytic oxidation of fuels or other reductants.